Abstract:

The probiotic application of dairy propionibacteria as well as their use in cheese technology implies
exposure to various environmental stresses, including acidic pH. The acid tolerance response (ATR) of
Propionibacterium freudenreichii was investigated. One strain present in Swiss-type cheese proved to
be acid-tolerant, since no lethal effect was observed during exposure at pH 3. Moreover, survival at pH 2
(acid challenge) was conferred by pre-exposure to a moderate acid stress (acid adaptation). This
adaptative response was triggered quickly, and showed a maximal efficiency upon exposure to pHs between 4
and 5. Stationary phase ATR and acid habituation were also demonstrated, and conferred increased survival
at pH 2 without pre-exposure. Exponentially-growing bacteria were partially protected towards acidity by
pre-exposure to other stresses (heat, starvation, but not hyperosmolarity). A comparative study of
different strains revealed that acid stress susceptibility is strain-dependent within this species.
Adaptation and survival at low pH is likely to determine the efficacy of a P. freudenreichii strain
both as a cheese starter and as a probiotic.